Acoustic methods for fracturing selected zones in a well bore

ABSTRACT

Following is disclosed acoustical well fracturing methods and apparatus whereby pressure fluctuations are generated in a well bore by pumping fluid through a first conduit to drive an acoustical oscillator coupled with an acoustical compliance that transmits the pressure fluctuations to a formation of the earth in a selected zone of the well bore. The well bore, which functions as a second conduit that contains the first conduit and the oscillator, returns fluid flow back toward a pump means. A variable restriction means such as a valve is used to adjust the back pressure in the well bore such that the maximum oscillated fluid pressure exceeds that pressure required for formation fracture. To achieve fracture only in the selected zone, acoustical isolation means are spaced above and below the acoustical oscillator to confine the pressure fluctuations to that zone. Also, in another embodiment the density of the fluid circulated in the well bore is selectively varied to achieve fracture in the selected zone.

United States Patent [191 Baker et al.

[451 Oct. 22, 1974 ACOUSTIC METHODS FOR FRACTURING SELECTED ZONES IN A WELL BORE [75] Inventors: Billy Eugene Baker; Edward M.

Galle, both of Houston, Tex.

[73] Assignee: Hughes Tool Company, Houston,

Tex.

221 Filed: Feb. 14, 1973 21 Appl.No.:332,471

[52] US. Cl. 166/249, 166/308 [51] Int. Cl... E21b 43/25, E21b 43/26, E21b 43/27 [58] Field of Search 166/308, 249

[56] References Cited UNITED STATES PATENTS 2,871,943 2/1959 Bodine 166/249 2,918,126 12/1959 Bodine.... 166/249 3,189,092 6/1965 Bodine 166/249 3,202,108 8/1965 Fly et al 166/308 X 3,322,196 5/1967 Bodine 166/249 3,323,592 6/1967 Brandon 166/308 3,602,311 8/1971 Whitsitt 166/308 3,743,017 Fast et al. 166/249 Primary Examiner-David H. Brown Attorney, Agent, or Firm-Robert A. Felsman [5 7] ABSTRACT Following is disclosed acoustical well fracturing methods and apparatus whereby pressure fluctuations are generated ina well bore by pumping fluid through a first conduit to drive an acoustical oscillator coupled with an acoustical compliance that transmits the pressure fluctuations to a formation of the earth in a selected zone of the well bore. The well bore, which functions as a second conduit that contains the first conduit and the oscillator, returns fluid flow back toward a pump means. A variable restriction means such as a valve is used to adjust the back pressure in the well bore such that the maximum oscillated fluid pressure exceeds that pressure required for formation fracture. To achieve fracture only in the selected zone, acoustical isolation means are spaced above and below the acoustical oscillator to confine the pressure fluctuations to that zone. Also, in another embodiment the density of the fluid circulated in the well bore is selectively varied to achieve fracture in the selected zone.

2 Claims, 7 Drawing Figures PAIENIED 061221974 PRESSURE I FORMATION FRACTURE PRESSURE SHEET 20F 2 SURFACE BACKPRESSURE REQUIRED TO INITIATE FRACTURE FRACTURED PRESSURE PRESSURE FLUCTUATION RANGE CIRCULATING FLUID PRESSURE ACOUSTIC METHODS FOR FRACTURING SELECTED ZONES IN A WELL BORE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates in general to hydraulic well fracturing, and in particular to the use of acoustical pressure flactuations in the fluid in a well bore to obtain fractures in selected zones.

2. Description of the Prior Art The prior art includes well fracturing methods and apparatus that utilize fluidic oscillators connected remotely if desired with the output of a fluid pump to generate pressure fluctuations that are acoustically coupled with a selcted zone in a well bore. In US. Pat. No. 3,405,770, Drilling Method and Apparatus Employing Pressure Variations in a Drilling Fluid, issued on Oct. 15, 1968, are disclosed drilling methods and apparatus that utilize acoustical apparatus for improv ing the penetration rate of an earth boring drill bit. Other related apparatus and methods are shown in US. Pat. No. 3,441,094, Drilling Methods and Apparatus Employing Out-of-Phase Pressure Variations in a Drilling Fluid, which issued on Apr. 29, 1969. In US. Pat. No. 3,520,362, Well Stimulation Method, are disclosed methods and apparatus for using a related acoustical system as a well stimulation method and means to improve the efficiency of the recovery of minerals from the earth.

SUMMARY OF THE INVENTION Zone selectivity in the above mentioned prior art acoustical well fracturing system is improved by the present invention through utilization of means to control at the surface the pressure of the fluid in the well bore. That is, a variable flow restriction means at the surface of the well is used to selectively control fluid back pressure to obtain pressure variations reaching a peak sufficient to achieve fracture at the selected zone. Alternatively, the density of the fluid in the well bore is selectively altered to produce facture at the selected zone.

Specifically, in any given well there is a minimum fluid pressure required to produce a formation fracture at a given depth. The maximum pressure in the fluid adjacent the zone to be fractured in a well utilizing the method of this invention is the sum of the average pressure in the fluid at that depth plus the maximum pressure fluctuation produced by an acoustic oscillator. If this maximum pressure is insufficient to cause the formation to fracture, the pressure will be increased with this invention by increasing the average fluid pressure adjacent the zone to be fractured. The average fluid pressure may be increased by: (l) controlling the restriction to flow of fluid returning from the annulus to create a back pressure on the entire system, (2) increasing the density of the circulating fluid, or (3) a combination of (1) and (2).

The invention therefore provides improved means and methods for more effectively and accurately producing formation fractures in a selected well bore zone that may be remotely located from an energy supplying pump means. This object is preferably accomplished through utilization of an acoustical oscillator connected with and driven by a pump means, an acoustical compliance coupled with the output of the acoustical oscillator, and a variable flow restriction means in the fluid return line. The fluid return line is connected with the acoustical compliance, and with the variable flow restriction means to selctively control the fluid pressure therein. In the preferred embodiment the oscillator consists of a bistable fluidic amplifier with positive feedback means to cause oscillation between two output legs. An acoustical coupler is utilized to invert the phase relationship of one leg such that the output of the oscillator is effectively connected with the acoustical compliance in the selected region or zone. To minimize energy dissipation in the return line and confine the pressure fluctuations to the selected zone, acoustical energy isolation means are spaced above and below the compliance to avoid the transmission of acoustical energy through the fluid return line. Seal means are utilized to direct fluid flow through a variable flow restric' tion means, which may for example be a manually or automatically controlled valve at the surface of the well that selectively varies back pressure in the system. From the variable flow restriction means, fluid may be returned to a fluid reservoir or sump and re-introduced to the pump means. In one embodiment weighting materials are added to the fluid in the well bore to increase the average pressure opposite a selected zone to be fractured. Thus by introducing pressure fluctuations opposite a selected zone and by controlling either the back pressure of the fluid in the well bore, or the density of the fluid in the well bore, or a combination of both, a fracture may be caused in a selected zone of the well.

Additional objects, features and advantages of the invention will become apparent in the following description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1A and 1B are side elevation views illustrating schematically acoustical vibration generation and control means used to accomplish the objects of the invention;

FIG. 2 is a side elevation view in longitudinal section showing a portion of the apparatus shown in FIG. 1;

FIG. 3 is a perspective view of the acoustical vibration generator assembly with a portion thereof broken away to expose its interior. The oscillator unit B is shown lifted from the normal position to add clarity to the drawing;

FIG. 4 is a plan view of a portion of the fluidic oscillator unit shown in perspective in FIG. 3;

FIG. 5 is a graph showing the variation with depth of the hydrostatic pressure in a well bore, the result when back pressure is increased in the fluid returning from the well bore, and the change in pressure if acoustically fluctuated; and

FIG. 6 is a graph, which shows the variation with depth of the hydrostatic pressure in a well bore, the result if the density of the fluid in the well bore is varied, and the change in pressure if acoustically fluctuated.

DESCRIPTION OF THE PREFERRED EMBODIMENT Initially will be described the preferred apparatus for generating acoustic vibrations in a well bore, for coupling these vibrations to the zone'to be fractured, for isolating the vibrations to the selected zone, and for controlling the average fluid pressure in the well bore adjacent a zone to be fractured. With reference initially g to FIG. 2 of the drawing, the letter A designates an acoustic vibration generator assembly which includes an oscillator unit B and a coupling device C. The coupling device C communicates with fluid adjacent a mineral producing region or zone F (see FIG. 1) to be fractured in a well bore. An upper resonator or acoustical filter D (see FIG. 2) is disposed above the generator assembly, and a lower resonator or acoustical filter E is disposed beneath the generator assembly. The apparatus described thusfar is supported by a first conduit or tubing string G (see FIG. 1) disposed inside a second conduit and a well bore H forming a portion of a fluid flow return line containing a variable flow restriction means such as the valve 1 which returns fluid to a fluid reservoir J. Seal means K between the first and second conduits prevents loss of fluid from the pump means L which supplies the first conduit G, the fluid oscillator unit B and coupling device C.

Describing the above components in greater detail, beginning from the top of FIG. 2, the numeral 11 designates a threaded coupling of a tubing member, which is received by a mating threaded portion 13 of a housing 15 which contains a resonator D having a cavity 17 defined by an interior cylindrical surface 19 of the housing 15, and exterior cylindrical surface 21 of an insert 23, a flange 25 on an upper region of the insert 23 and a radial shoulder 27 on a sub 29 secured by threads 31 to housing 15 and by threads 33 to an upper portion of a housing 35 of the acoustic vibration generator assembly A. Since the insert 23 is removable: in this instance, suitable seal means are used, as indicated in FIG. 2, to prevent fluid flow to or from cavity 17 except through apertures 37 extending obliquely through the sub 29. The word tubing is used broadly to encompass any conduit, usually an elongated tubular member.

Housing 35 of the generator assembly A contains the acoustic coupling device C and the oscillator unit B, both of which may be of the type described in US. Pat. No. 3,405,770, Drilling Methods and Apparatus Employing Pressure Variations in a Drilling Fluid," issued Oct. 15, 1968. As described in that patent, the coupling device C is tuned to the operating frequency of the oscillator unit B, and has one or more exit ports 39 extending through exterior surface 41 of the housing into communication with the fluid surrounding a small diameter sub 47. The invention is not limited to the specific forms of oscillators and coupling devices described in the above mentioned patent, but encompasses, at least in its broadest aspects, other suitable forms of oscillator units and coupling devices, although the above fluidic (i.e., containing no moving mechanical components) devices appear to be most advantageous since they eliminate moving mechanical parts.

The lower region of the housing 35 of the generator assembly A has a small diameter region 43 connected by threads 45 to a similarly small diameter sub 47 which has its lower region secured by threads 49 to the housing 51 which contains lower resonator or filter E, which is one form of acoustic vibration isolator means. An axial bore 53 extends downward through the upper resonator D, sub 29, generator assembly A, coupling device C and sub 47. Bore 53 terminates in this instance at the top of housing 51. Howver, inother embodiments, this bore can communicate with passages for the flow of fluid therethrough.

As shown in FIG. 2, one or more apertures 55 are formed obliquely in housing 51 to communicate with the annulus (that space between the housing and the wall of the'borehole) and a cavity 57 formed on a lower region of the housing by a sleeve 59 secured by threads 61 to the housing and by a plug 63 secured by threads 65 to the sleeve 59. The relative sizes of the apertures 55 and cavity 57 are selected such that the resonator is tuned substantially to the operating frequency of the oscillator unit B.

The volume of fluid between the wall of the bore hole and the exterior surface of sub 47 and the small diameter regions of housing 35 and 51 defines an exterior acoustic tank or compliance 66 opposite the region, medium or zone to receive acoustic vibrations, said compliance having dimensions correlated with the dimensions of the apertures 39 and cavity 69 of the coupling device C to couple the oscillator unit with the acoustic load.

The axial bore 53 is an extension of the first conduit G shown in FIG. 1 which receives fluid from the pump means L connected with the fluid reservoir J. The first conduit G extends inside a second conduit H, which is sealed from the first conduit by the seal means K. The fluid line 70 extends from the second conduit H and forms a part of a fluid flow return line that includes a variable flow restriction means I, which in this instance is a valve that may be opened or closed to control the static pressure in the system. Thus, by opening or closing the valve, selective control is maintained over the average pressure in the fluid return line, over the average pressure in the acoustical compliance, and over the pressure drop across the acoustical oscillator.

Referring now to FIGS. 3 and 4, the acoustic vibration generator assembly A includes a fluidic oscillator in that it has no moving mechanical components. It is a high gain, bi-stable fluidic amplifier with positive feedback to cause oscillation of the bi-stable unit. The coupling device B couples the output of the acoustic vibration oscillator with the drilling fluid located in the acoustic compliance, cavity or tank 66 as shown in FIG. 2.

The preferred oscillator configuration is shown in FIGS. 3 and 4, wherein the oscillator is designated by the letter B. Fluid flowing through the first conduit G and the associated passage 53 shown in FIG. 2 is diverted through a supply port or inlet passage 163 of the oscillator. From input 163, the fluid flows from a power nozzle 167 and alternatively flows into receiver channels 185 and 187. This alternating flow results from the positive feedback effected by feedback channels 193, 195; feedback ports 197, 199; apertures 159, cavities formed in the axial bores 143, 145; apertures 157, 158; control ports 179, 181; passages 175, 177; and control nozzles 171, 173. During each half cycle of oscillation, a majority of the fluid entering the receiver channels 185 or 187 flows into either diffuser channel 189 or 191 and to the outlet 201 or 203 of the acoustic vibration oscillator. The output of the diffuser channel 191 feeds aperture 213, tube 217 and aperture 39, which together with the fluid therein constitute an acoustic inertance. Aperture or passage 39 communicates with the fluid in the acoustic tank or compliance. The output of the diffuser channel 189 feeds aperture 215 and tube 223, which constitute another acoustic inertance. Tube 223 terminates within annular cavity 69, which constitutes another acoustic compliance.

Passage or aperture 39 is an acoustic inertance communicating between annular cavity 69 and the acoustic tank or compliance. To improve the reliability of oscillation on-set under high pressure conditions, it is advantageous to insert flow restriction in diffuser channels 189 and 191. However, the openings of the restrictions should be made as large as possible to minimize power loss. Suitable dimensioning of all the acoustical elements within the acoustical coupling circuitry accomplishes three objectives: (l) proper matching of the output impedance of the oscillator A with the dissipative load on the circuit; (2) effective phase inversion of the vibrations in one of the output legs of the oscillator A; and (3) the provision of a high Q system. Hence, the acoustic generator means may be utilized to effect large pressure variations at selected frequencies in the acoustic comliance or tank 66.

In operation pump L is activated to draw fluid from the fluid reservoir J and force it under pressure through the tubing string or first conduit G, located partially inside the well bore or second conduit H, which contains at a selected location the acoustic vibration generator assembly A and acoustic compliance 66 opposite region zone F of the formation to be fractured. Fluid therefore flows from the tubing string G into the axial opening 53 (see FIG. 2) to feed the oscillator unit B which generates acoustic energy. This acoustic energy is transmitted by acoustic coupling device C and the exit ports 39 and 39' into the acoustic tank or compliance 66. Fluid then returns to the surface of the well bore in the annulus of the well bore. Consequently, acoustic energy is transmitted to the earth. The previously described acoustic energy isolation means or acoustic filters or resonators D and E prevent the dissipation of substantial quantities of acoutic energy up ward or downward in the annulus. The length of zone F in FIG. 1 may be varied by inserting different lengths of subs between the housing 35 of the generator assembly A and the housing 51 of the lower resonator E to vary the length of the acoustically treated zone. Acoustic energy will normally travel both upward and downward through the well bore but is effectively prevented from doing so in this instance by the use of the isolator means. The resonators D and E are used as side branches with inlets at points one quarter wave length above and below the acoustic tank 66. This effectively causes the acoustic impedance looking into the annulus from acoustic tank 66 to be very high, thus preventing substantial transmission of acoustical power either up or down the annulus.

Fluid flow returns up the annulus between the tubing string or first conduit G and the well bore or second conduit H toward the pump L. A seal means K, which may be in the form of a conventional blow-out preventer pipe ram, is inserted between the first and second conduits G and H to cause fluid to return to the variable flow restriction means, in this instance valve I located in the return line 70. By varying the setting of this valve, the back pressure in the acoustic circuit and in the well bore may be maintained at a selected average pressure. The flow restriction and back pressure control are beneficial in preventing cavitation in the oscillator; otherwise substantial damage to the system may result. In addition, efficient operation of the oscillator is not achieved unless cavitation is prevented. Further, if peak-to-peak pressure amplitudes of 1,500 psi are to be utilized, for example, the average static back pressure in the vicinity of the oscillator should be at least 1,500 psi and preferably somewhat above this figure or otherwise the desired peak-to-peak pressure variations cannot be obtained.

The beneficial effects achieved by the invention as a well fracturing method and apparatus is demonstrated in FIG. 5 wherein the numeral 301 represents the abscissa of a graph designating the pressure of the fluid in a well bore. The numeral 303 represents the ordinate of this graph upon which is shown the depth of a well bore. The line designated by the numeral 305 is a plot of the pressure required to initiate a fracture in the well bore. This formation fracture pressure generally increases with depth.

In the graph of FIG. 5, the numeral 307 designates a line representing the pressure of the fluid circulating through the well bore and shows how this pressure increases with depth in the well bore. When using the invention, the actual range of pressures existing in the fluid adjacent a zone to be fractured is indicated by the short horizontal line 309, which represents the peak-topeak. pressure fluctuation range produced by the acoustic vibration generator assembly previously described. The resulting pressure fluctuations cause the pressure to vary equally above and below line 307. The zone to be fractured is indicated by the letter F on the ordinate 303. Since the sum of the circulating fluid pressure and the fluctuating pressure at this depth is less than the formation fracture pressure, fracturing will not occur. Increasing the back pressure on the system through adjustment of the valve means or variable flow restriction means I increases the pressures throughout the system uniformly, shifting the line 309 to the right. Back pressure in the system is increased slowly until the pressure of the circulating fluid assumes the position indicated by the dashed line 311 of the graph. As shown, the maximum pressure achieved through addition of the circulating pressure and the peak positive pressures achieved with fluctuations of the oscillator is now equal to the formation fracture pressure depicted by the line 305. Hence, fracturing can occur. Since the pressure fluctuations are isolated to the region of zone F by the previously described isolation means, at zone F fluid pressures are great enough to induce fractures. Otherwise, fracture, might occur between the depths represented by the lines 309 and 313. Line 313 represents the depth that casing has been set in the particular well under consideration.

In FIG. 6 are shown the same abscissa 301, pressure, ordinate 303, depth, and the zone F where the formation is to be fractured. Line 307 represents the fluid pressure when plotted against depth, and the line 309 represents the range of the pressure fluctuations generated by the oscillator. Also, the line 305 represents formation fracture pressure plotted against depth. With a low density fluid, the maximum pressure at zone F may be less than the formation fracture pressure at that depth and fracturing will not occur, as indicated by the line 307 in FIG. 6. Increasing the density of the circulating fluid by the addition of weighting material to the mud will increase the fluid pressure as shown by the dashed line 315. Hence, fluid density may be increased until the sum of the circulating fluid pressure and the fluctuating pressure adjacent the zone to be fractured is large enough to induce fracture. This condition is represented by the intersection of line 309' with the formation fracture pressure line 305 shown in FIG. 6.

Increasing the back pressure and increasing the circulating fluid density may be combined to induce fractures in the formation. Generally, it is more economical to use higher back pressure than increased circulating fluid density to initiate fracture. However, caution must be exercised not to increase back pressure to the point of initiating a fracture outside of zone F immediately below the casing, as for example, when the formation back pressure is increased until it is greater than the formation fracture pressure immediately below the easing. Thus by properly combining selected back pressures and fluid densities, fracture initiations may be confined to a selected zone.

While FIGS. and 6 indicate that the zones to be fractured are below the bottom of the casing, it should be understood that the invention is not restricted to use in open formations. It may be used to fracture formations through perforations in a casing.

It should be apparent from the foregoing that an invention has been provided having significant advantages. Through alteration and control the back pressure of the circulating fluid in a well bore and/or by controlling and adjusting the density of this fluid, fractures may be located in a selected zone through utilization of apparatus of the type previously described.

While the invention has been described in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes and modifications without departing from the spirit thereof. The specific forms of the apparatus such as for example the oscillator, the coupling device, the connecting conduits, the variable restriction means and the pump means may be varied widely by those of average skill in the art to accomplish the objects and advantages of the invention.

We claim: 1. An acoustical method for fracturing selected zones in a well bore comprising the steps of:

pumping a fluid down a well bore through a first conduit;

oscillating the pressure in said fluid within the well bore;

coupling the resulting fluid pressure oscillations with the bore hole wall at a selected depth;

isolating the fluid pressure oscillations to a selected zone to be fractured;

returning the fluid flow between the first conduit and a second conduit formed at least partially by the wall of the well bore;

increasing the density of the fluid to control the fluid pressure in the well bore to initiate fracture in the selected zone.

2. An acoustic method for fracturing selected zones in a well bore comprising the steps of:

pumping a fluid down a well bore through a first conduit;

oscillating the pressure in said fluid within the well bore;

coupling the resulting fluid pressure oscillations with the well bore wall in a selected zone;

isolating the fluid pressure oscillations to a selected region adjacent the selected formation to be fractured;

returning the fluid flow between the first conduit and a second conduit formed at least partially by the wall of the well bore;

controlling from the surface of the earth the restriction to the flow of fluid from said second conduit and increasing the density of the fluid to control the fluid pressure in the well bore to initiate fracture in the selected zone. 

1. An acoustical method for fracturing selected zones in a well bore comprising the steps of: pumping a fluid down a well bore through a first conduit; oscillating the pressure in said fluid within the well bore; coupling the resulting fluid pressure oscillations with the bore hole wall at a selected depth; isolating the fluid pressure oscillations to a selected zone to be fractured; returning the fluid flow between the first conduit and a second conduit formed at least partially by the wall of the well bore; increasing the density of the fluid to control the fluid pressure in the well bore to initiate fracture in the selected zone.
 2. An acoustic method for fracturing selected zones in a well bore comprising the steps of: pumping a fluid down a well bore through a first conduit; oscillating the pressure in said fluid within the well bore; coupling the resulting fluid pressure oscillations with the well bore wall in a selected zone; isolating the fluid pressure oscillations to a selected region adjacent the selected formation to be fractured; returning the fluid flow between the first conduit and a second conduit formed at least partially by the wall of the well bore; controlling from the surface of the earth the restriction to the flow of fluid from said second conduit and increasing the density of the fluid to control the fluid pressure in the well bore to initiate fracture in the selected zone. 